is service tax mandatory, if we registered with st-2 and turnover is less than 10 lakh ?

If i registered with Service Tax registration (ST-2), and my turnover is below 10 lac per annum. Is it necessary to pay Service tax? Actually i registered as a proprietorship and used ST-2 for only Business name. Somebody said if you registered with service tax then its mandatory to pay 12.36% service tax even your turnover is below 10 lac annually.